German patent document DE 10 2005 062 263 A1 discusses a method and a device for detecting objects in the immediate surroundings of a vehicle, in which the objects are detected by multiple distance sensors. A so-called signal collision occurs when one sensor detects a signal emitted by another sensor, so that the propagation time of the ultrasonic signal is calculated incorrectly, resulting in an erroneous distance measurement. To prevent the signal collision, it is provided that the detected signals are to be filtered as a function of the propagation time. The distance sensors detect only signals within a relatively narrow time window. In addition to that, a signal coding may take place with the aid of a signal pulse width modulation or an amplitude modulation. Specifying a relatively narrow time window entails safety risks, namely that signals are not detected, despite having been reflected. This risk is not remedied by specifying the modulation.
Patent document EP 1 105 749 B1 discusses a method and a device for detecting objects, in particular as a parking assistance device in a motor vehicle, misidentification of obstacles in the event that two or multiple transducers on a vehicle are transmitting at the same time being avoided by the fact that the distance sensors receive a chronologically variable identifier, so that the received signals may be assigned to sources reliably and unambiguously.